Weinheimer, C., Degenddag, B., Bleile, A., Bonn, J., Bornschein, L., Kazachenko, O., Kovalík, A., Otten, E. W. High precision measurement of the tritium .beta.spectrum near its endpoint and upper limit on the neutrino mass. Physics Letters. B. 1999, 460(-), 219-226. ISSN 0370-2693. E-ISSN 1873-2445.